Common-rail fuel injection systems have been in general use for many years with gasoline-fueled internal combustion engines. Lately, common-rail systems have been used with diesel engines. Common-rail systems typically use a fuel pump to provide pressurized fuel, most often in a liquid form, to the fuel rail. In turn, injectors which are plugged into or otherwise operatively connected with a fuel rail, furnish fuel to the engine's cylinders. When compressed gas is used for an internal combustion engine fuel, the high pressure gas in the fuel tank is stepped down in pressure using one or more pressure regulators, and a fuel pump is generally not needed.
Given the comparatively low density of gaseous fuel, as compared with liquid fuels, it is necessary to handle a much higher volume of gaseous fuel than would otherwise be the case were liquid fuel to be used. This results in the need for gaseous fuel injectors to have a very large range of authority as compared with injectors used for injecting liquid fuels.
The amount of gas injected per injection event by gaseous fuel injectors may be controlled not only by the pulse width furnished to the injector solenoid, but also by the common-rail pressure. As shown in FIG. 3, as common-rail pressure increases, fuel delivery increases concomitantly for equal duration injection events. A problem may arise, however, where it is desired to transition from a higher injection rate to a lower mass injection per injection event. Although less fuel may be injected by decreasing the injector pulse width, this does not always produce desirable results at low flows because the desired amount of fuel may be less than the range of authority of the injector at the pressure available within the common-rail. This is a problem where it is desired to inject small amounts of fuel, given that the fuel rail represents a relatively larger volume, sometimes at a higher pressure than is desired.
When the pressure within the fuel rail is higher than desired, it is necessary to remove some of the gaseous fuel from the rail other than by running the fuel through the injectors. Although it would be possible to simply release the fuel to the atmosphere, the negative impact on fuel economy is an obvious deterrent to such a strategy. The problem of how to reintroduce a fuel to the fuel system is solved by the present invention. By removing some of the fuel from the fuel rail and by recompressing the fuel and injecting fuel into the fuel system upstream of the common-rail, the rail pressure may be reduced to the desired value without wasting fuel. The present inventive system solves the problems that prior art fuel systems experience in handling compressed gas while at the same time achieving a dynamic range of response of injectors by varying the common-rail fuel pressure.